fix #280 nullpointer in gui while simulation setup

The problem occurred because the OnlineVisualization model is setup during gui startup but was only valid after the first call to OnlineVisualization#postUpdate where the DrawData object is created. Only then was the OnlineVisualization valid and capable of answering all callbacks.

This commit will fix the problem but does not fix the design the problem. The OnlineVisualization is still created at gui startup but all callbacks to the model are removed util the OnlineVisualization model is fully initialized.